This invention relates to hydraulic retarders and controls therefore, and more particularly, to controls for filling an accumulator while the retarder is operative or inoperative, for preventing fluid reduction in the accumulator between retard requests, and for limiting the inlet pressure at the retarder.
The prior art hydraulic retarder and control mechanisms utilize an accumulator and a friction device simultaneously. The friction device initiates retardation, thereby compensating for the fill time required to dump the accumulator to the retarder. These devices permit uncontrolled fluid flow from the accumulator to the retarder, and therefore, substantially empty the accumulator with each retard operation, thereby requiring that the accumulator be completely refilled between retard operations. As a general rule, to accommodate the accumulator dumping and filling process, the displacement of the control pump for the transmission is increased.
As an alternative, retarder systems have employed a secondary pump, generally driven by the transmission output to provide pressure for the retarder and refilling of the accumulator. The secondary pump is usually in continuous flow with the retarder during the retarding operation. In either event, the requirement of increased oil supply from the control pump reduces the overall efficiency of the transmission. Also, due to the time lag between retard request and retard operation, a friction device is necessary for complete retarder control. This, of course, adds to the overall cost and complexity of the hydraulic retarder.